/*
 *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#ifndef API_ARRAY_VIEW_H_
#define API_ARRAY_VIEW_H_

#include <algorithm>
#include <array>
#include <type_traits>

#include "checks.h"
#include "type_traits.h"

namespace rtc {

// tl;dr: rtc::ArrayView is the same thing as gsl::span from the Guideline
//        Support Library.
//
// Many functions read from or write to arrays. The obvious way to do this is
// to use two arguments, a pointer to the first element and an element count:
//
//   bool Contains17(const int* arr, size_t size) {
//     for (size_t i = 0; i < size; ++i) {
//       if (arr[i] == 17)
//         return true;
//     }
//     return false;
//   }
//
// This is flexible, since it doesn't matter how the array is stored (C array,
// std::vector, rtc::Buffer, ...), but it's error-prone because the caller has
// to correctly specify the array length:
//
//   Contains17(arr, arraysize(arr));     // C array
//   Contains17(arr.data(), arr.size());  // std::vector
//   Contains17(arr, size);               // pointer + size
//   ...
//
// It's also kind of messy to have two separate arguments for what is
// conceptually a single thing.
//
// Enter rtc::ArrayView<T>. It contains a T pointer (to an array it doesn't
// own) and a count, and supports the basic things you'd expect, such as
// indexing and iteration. It allows us to write our function like this:
//
//   bool Contains17(rtc::ArrayView<const int> arr) {
//     for (auto e : arr) {
//       if (e == 17)
//         return true;
//     }
//     return false;
//   }
//
// And even better, because a bunch of things will implicitly convert to
// ArrayView, we can call it like this:
//
//   Contains17(arr);                             // C array
//   Contains17(arr);                             // std::vector
//   Contains17(rtc::ArrayView<int>(arr, size));  // pointer + size
//   Contains17(nullptr);                         // nullptr -> empty ArrayView
//   ...
//
// ArrayView<T> stores both a pointer and a size, but you may also use
// ArrayView<T, N>, which has a size that's fixed at compile time (which means
// it only has to store the pointer).
//
// One important point is that ArrayView<T> and ArrayView<const T> are
// different types, which allow and don't allow mutation of the array elements,
// respectively. The implicit conversions work just like you'd hope, so that
// e.g. vector<int> will convert to either ArrayView<int> or ArrayView<const
// int>, but const vector<int> will convert only to ArrayView<const int>.
// (ArrayView itself can be the source type in such conversions, so
// ArrayView<int> will convert to ArrayView<const int>.)
//
// Note: ArrayView is tiny (just a pointer and a count if variable-sized, just
// a pointer if fix-sized) and trivially copyable, so it's probably cheaper to
// pass it by value than by const reference.

    namespace impl {

// Magic constant for indicating that the size of an ArrayView is variable
// instead of fixed.
        enum : std::ptrdiff_t {
            kArrayViewVarSize = -4711
        };

// Base class for ArrayViews of fixed nonzero size.
        template<typename T, std::ptrdiff_t Size>
        class ArrayViewBase {
            static_assert(Size > 0, "ArrayView size must be variable or non-negative");

        public:
            ArrayViewBase(T *data, size_t size) : data_(data) {}

            static constexpr size_t size() { return Size; }

            static constexpr bool empty() { return false; }

            T *data() const { return data_; }

        protected:
            static constexpr bool fixed_size() { return true; }

        private:
            T *data_;
        };

// Specialized base class for ArrayViews of fixed zero size.
        template<typename T>
        class ArrayViewBase<T, 0> {
        public:
            explicit ArrayViewBase(T *data, size_t size) {}

            static constexpr size_t size() { return 0; }

            static constexpr bool empty() { return true; }

            T *data() const { return nullptr; }

        protected:
            static constexpr bool fixed_size() { return true; }
        };

// Specialized base class for ArrayViews of variable size.
        template<typename T>
        class ArrayViewBase<T, impl::kArrayViewVarSize> {
        public:
            ArrayViewBase(T *data, size_t size)
                    : data_(size == 0 ? nullptr : data), size_(size) {}

            size_t size() const { return size_; }

            bool empty() const { return size_ == 0; }

            T *data() const { return data_; }

        protected:
            static constexpr bool fixed_size() { return false; }

        private:
            T *data_;
            size_t size_;
        };

    }  // namespace impl

    template<typename T, std::ptrdiff_t Size = impl::kArrayViewVarSize>
    class ArrayView final : public impl::ArrayViewBase<T, Size> {
    public:
        using value_type = T;
        using const_iterator = const T *;

        // Construct an ArrayView from a pointer and a length.
        template<typename U>
        ArrayView(U *data, size_t size)
                : impl::ArrayViewBase<T, Size>::ArrayViewBase(data, size) {
            RTC_DCHECK_EQ(size == 0 ? nullptr : data, this->data());
            RTC_DCHECK_EQ(size, this->size());
            RTC_DCHECK_EQ(!this->data(),
                          this->size() == 0);  // data is null iff size == 0.
        }

        // Construct an empty ArrayView. Note that fixed-size ArrayViews of size > 0
        // cannot be empty.
        ArrayView() : ArrayView(nullptr, 0) {}

        ArrayView(std::nullptr_t)  // NOLINT
                : ArrayView() {}

        ArrayView(std::nullptr_t, size_t size)
                : ArrayView(static_cast<T *>(nullptr), size) {
            static_assert(Size == 0 || Size == impl::kArrayViewVarSize, "");
            RTC_DCHECK_EQ(0, size);
        }

        // Construct an ArrayView from a C-style array.
        template<typename U, size_t N>
        ArrayView(U (&array)[N])  // NOLINT
                : ArrayView(array, N) {
            static_assert(Size == N || Size == impl::kArrayViewVarSize,
                          "Array size must match ArrayView size");
        }

        // (Only if size is fixed.) Construct a fixed size ArrayView<T, N> from a
        // non-const std::array instance. For an ArrayView with variable size, the
        // used ctor is ArrayView(U& u) instead.
        template<typename U,
                size_t N,
                typename std::enable_if<
                        Size == static_cast<std::ptrdiff_t>(N)>::type * = nullptr>
        ArrayView(std::array<U, N> &u)  // NOLINT
                : ArrayView(u.data(), u.size()) {}

        // (Only if size is fixed.) Construct a fixed size ArrayView<T, N> where T is
        // const from a const(expr) std::array instance. For an ArrayView with
        // variable size, the used ctor is ArrayView(U& u) instead.
        template<typename U,
                size_t N,
                typename std::enable_if<
                        Size == static_cast<std::ptrdiff_t>(N)>::type * = nullptr>
        ArrayView(const std::array<U, N> &u)  // NOLINT
                : ArrayView(u.data(), u.size()) {}

        // (Only if size is fixed.) Construct an ArrayView from any type U that has a
        // static constexpr size() method whose return value is equal to Size, and a
        // data() method whose return value converts implicitly to T*. In particular,
        // this means we allow conversion from ArrayView<T, N> to ArrayView<const T,
        // N>, but not the other way around. We also don't allow conversion from
        // ArrayView<T> to ArrayView<T, N>, or from ArrayView<T, M> to ArrayView<T,
        // N> when M != N.
        template<
                typename U,
                typename std::enable_if<Size != impl::kArrayViewVarSize &&
                                        HasDataAndSize<U, T>::value>::type * = nullptr>
        ArrayView(U &u)  // NOLINT
                : ArrayView(u.data(), u.size()) {
            static_assert(U::size() == Size, "Sizes must match exactly");
        }

        template<
                typename U,
                typename std::enable_if<Size != impl::kArrayViewVarSize &&
                                        HasDataAndSize<U, T>::value>::type * = nullptr>
        ArrayView(const U &u)  // NOLINT(runtime/explicit)
                : ArrayView(u.data(), u.size()) {
            static_assert(U::size() == Size, "Sizes must match exactly");
        }

        // (Only if size is variable.) Construct an ArrayView from any type U that
        // has a size() method whose return value converts implicitly to size_t, and
        // a data() method whose return value converts implicitly to T*. In
        // particular, this means we allow conversion from ArrayView<T> to
        // ArrayView<const T>, but not the other way around. Other allowed
        // conversions include
        // ArrayView<T, N> to ArrayView<T> or ArrayView<const T>,
        // std::vector<T> to ArrayView<T> or ArrayView<const T>,
        // const std::vector<T> to ArrayView<const T>,
        // rtc::Buffer to ArrayView<uint8_t> or ArrayView<const uint8_t>, and
        // const rtc::Buffer to ArrayView<const uint8_t>.
        template<
                typename U,
                typename std::enable_if<Size == impl::kArrayViewVarSize &&
                                        HasDataAndSize<U, T>::value>::type * = nullptr>
        ArrayView(U &u)  // NOLINT
                : ArrayView(u.data(), u.size()) {}

        template<
                typename U,
                typename std::enable_if<Size == impl::kArrayViewVarSize &&
                                        HasDataAndSize<U, T>::value>::type * = nullptr>
        ArrayView(const U &u)  // NOLINT(runtime/explicit)
                : ArrayView(u.data(), u.size()) {}

        // Indexing and iteration. These allow mutation even if the ArrayView is
        // const, because the ArrayView doesn't own the array. (To prevent mutation,
        // use a const element type.)
        T &operator[](size_t idx) const {
            RTC_DCHECK_LT(idx, this->size());
            RTC_DCHECK(this->data());
            return this->data()[idx];
        }

        T *begin() const { return this->data(); }

        T *end() const { return this->data() + this->size(); }

        const T *cbegin() const { return this->data(); }

        const T *cend() const { return this->data() + this->size(); }

        ArrayView<T> subview(size_t offset, size_t size) const {
            return offset < this->size()
                   ? ArrayView<T>(this->data() + offset,
                                  std::min(size, this->size() - offset))
                   : ArrayView<T>();
        }

        ArrayView<T> subview(size_t offset) const {
            return subview(offset, this->size());
        }
    };

// Comparing two ArrayViews compares their (pointer,size) pairs; it does *not*
// dereference the pointers.
    template<typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
    bool operator==(const ArrayView<T, Size1> &a, const ArrayView<T, Size2> &b) {
        return a.data() == b.data() && a.size() == b.size();
    }

    template<typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
    bool operator!=(const ArrayView<T, Size1> &a, const ArrayView<T, Size2> &b) {
        return !(a == b);
    }

// Variable-size ArrayViews are the size of two pointers; fixed-size ArrayViews
// are the size of one pointer. (And as a special case, fixed-size ArrayViews
// of size 0 require no storage.)
    static_assert(sizeof(ArrayView<int>) == 2 * sizeof(int *), "");
    static_assert(sizeof(ArrayView<int, 17>) == sizeof(int *), "");
    static_assert(std::is_empty<ArrayView<int, 0>>::value, "");

    template<typename T>
    inline ArrayView<T> MakeArrayView(T *data, size_t size) {
        return ArrayView<T>(data, size);
    }

// Only for primitive types that have the same size and aligment.
// Allow reinterpret cast of the array view to another primitive type of the
// same size.
// Template arguments order is (U, T, Size) to allow deduction of the template
// arguments in client calls: reinterpret_array_view<target_type>(array_view).
    template<typename U, typename T, std::ptrdiff_t Size>
    inline ArrayView<U, Size> reinterpret_array_view(ArrayView<T, Size> view) {
        static_assert(sizeof(U) == sizeof(T) && alignof(U) == alignof(T),
                      "ArrayView reinterpret_cast is only supported for casting "
                      "between views that represent the same chunk of memory.");
        static_assert(
                std::is_fundamental<T>::value && std::is_fundamental<U>::value,
                "ArrayView reinterpret_cast is only supported for casting between "
                "fundamental types.");
        return ArrayView<U, Size>(reinterpret_cast<U *>(view.data()), view.size());
    }

}  // namespace rtc

#endif  // API_ARRAY_VIEW_H_
